The present invention relates to a method for setting the tension of flexible members and more particularly to a method for prestretching and tensioning of drive belts such as those used to drive accessory equipment on internal combustion engines.
Flexible members in the form of V-belts, toothed belts, chains, etc. for example are commonly used in a wide variety of applications to transfer driving power from one source to another. In each of these applications there is provided a pair of rotatable members between which the interconnecting flexible member extends, one being the driven member and the other being the driving member. Typically, one of these rotatable members will be supported in such a manner as to be linearly adjustably movable with respect to the other rotatable member. In many of these applications the ability of the driving member to transmit the forces to the driven member is directly dependent upon the frictional engagement between the flexible member and the driving and driven members respectively. The ability of this frictional engagement to transmit the required driving force generally varies directly with the tension exerted on the flexible member. Thus, it is necessary to insure that a minimum amount of tension is exerted on the interconnecting flexible member, however, care must be taken to avoid over-tensioning the flexible member as this may result in excessive wear of the flexible member and/or overloading of the bearings supporting the driven and/or driving members. Even in those applications wherein the flexible member is in the form of a chain or toothed belt and hence not solely dependent on frictional engagement to transmit driving forces, it is still important to insure that a minimum amount of tension is exerted thereon in order to insure proper engagement of the links or teeth with the rotatable members.
It should also be noted that many such flexible drive members experience a period of stretching or elongation during the initial period of use which may result in a significant reduction in the tension exerted thereon. This is particularly true with respect to V-belts and results from both seating of the belt within the pulley grooves as well as a stretching of the belt material. In order to overcome the potential problems that may result from such a reduction in the desired tension as a result of the stretching, it has become common practice to subject the new V-belt to a prestretching process. This typically involves subjecting the V-belt to a tension in excess of that to which it is ultimately to be set and thereafter relaxing the tension on the belt and repeating this cycle several times before setting the tension to the predetermined level. This same process may also be desirable in conjunction with installation of other types of flexible drive members as well.
In asembly line operations this prestretching and final tensioning of the various drive belts incorporated on the apparatus being assembled can become a relatively time consuming and costly process. Further, in those situations where the prestretching and tensioning is performed in a manual fashion, the degree of prestretching as well as the final tensioning of the belt may vary significantly from one installation to the next. This is also possible when presently available automatic equipment is used which merely applies a predetermined force to one of the driving or driven members via a pneumatic or hydraulic cylinder because excessive friction in the movable mounting or support of the driving or driven member may prevent the desired level of tension being applied to the flexible member.
The present invention, however, provides a method and apparatus for carrying out the method whereby V-belts and other flexible interconnecting members may be automatically subjected to both a prestretching and final tensioning process relatively easily and conveniently. The present invention includes a sensor assembly which may be attached to the flexible member at a convenient location and operates to continuously sense the tension to which the member is subjected. The sensor signal is supplied to central control means which operates to cycle force generating means which will vary the relative positions of the driving and driven members so as to thereby selectively increase and decrease the tension on the flexible member. The central control means may be programmed for any number of cyclic operations to properly prestretch and seat the flexible member and thereafter set the tension thereon to the final desired level. Suitable torque means such as an air wrench or the like are also connected to the central control means which will act to prevent operation thereof until such time as the desired cycling and final setting of the tension is completed. Thus, because the central control means continuously monitors and controls the actual tension exerted on the flexible member via the sensor and force generating means, each flexible member will be installed in substantially identical manner thereby eliminating the need for subsequent re-adjustment of the flexible member.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.